No safe threshold for exposures to fine particulates, ground-level ozone

Action Points

Long-term exposure to airborne fine particulate matter (PM2.5) and ozone was associated with an increased risk of death among Medicare beneficiaries, even when exposure levels were below national air quality standards.

Note that there was no appreciable level below which mortality risk tapered off, and thus no "safe" level of PM2.5.

Long-term exposure to airborne fine particulate matter (PM2.5) and ozone was associated with an increased risk of death among Medicare beneficiaries, even for exposures below National Ambient Air Quality Standards, reported researchers.

Analysis of more than 60 million Medicare beneficiaries found that each increase in PM2.5 of 10 µg/m3 was associated with a 7.3% increase in all-cause mortality (95% CI 7.1%-7.5%), reported Qian Di, MS, of Harvard University, and colleagues in the New England Journal of Medicine.

"If we could reduce PM2.5 level by just 1 µg/m3 nationwide, we could save about 12,000 lives just among people above 65 years old. The health benefit could be even larger in the overall population," Di told MedPage Today.

"There was no appreciable level below which mortality risk tapered off, and thus no 'safe' level of PM2.5. More importantly, we found significant harmful effects of air pollution below the current National Ambient Air Quality Standards for PM2.5."

Writing in an accompanying editorial, titled "Air Pollution Still Kills," Rebecca E. Berger, MD, of Massachusetts General Hospital in Boston, and colleagues said, "Owing to the large size of the cohort, [the researchers] were able to perform robust subgroup analyses and identified greater risks of death associated with air pollutants among blacks and Medicaid-eligible populations; moreover, these groups were more likely to be exposed to higher pollutant levels.

"The findings stress the need for tighter regulation of air-pollutant levels, including the imposition of stricter limits on levels of PM2.5."

Di and colleagues constructed an open cohort of 60,925,443 Medicare beneficiaries in the continental U.S. with 460,310,521 person-years of follow-up. The team analyzed 2002-2012 data from the Medicare beneficiary denominator file from the Centers for Medicare and Medicaid Services on all beneficiaries age 65 and older with all-cause mortality as the outcome. This included the date of death (up to Dec. 31, 2012), age at year of Medicare entry, year of entry, sex, race, ZIP code of residence, and Medicaid eligibility (a proxy for low socioeconomic status).

Annual averages of fine particulate matter PM2.5 and ozone were estimated according to the ZIP code of residence for each enrollee via previously validated prediction models.

Using a two-pollutant Cox proportional-hazards model that controlled for demographic characteristics, Medicaid eligibility, and area-level covariates, the researchers estimated the risk of death associated with exposure to increases of 10 μg per cubic meter for PM2.5 and 10 parts per billion (ppb) for ozone.

Annual average PM2.5 concentrations across the U.S. ranged from 6.21 to 15.64 μg per cubic meter (fifth and 95th percentiles, respectively), and the warm-season average ozone concentrations ranged from 36.27 to 55.86 ppb (fifth and 95th percentiles, respectively). The highest PM2.5 concentrations were in California and the eastern and southeastern U.S., while the Mountain region and California had the highest ozone concentrations.

For each 1 ppb increase in ozone, the associated mortality rate increase was about 1.1% (95% CI 1.0-1.2), Di et al reported. Similarly, a reduction in ozone level of just 1 ppb nationwide could save 1,900 elderly lives each year, the team wrote.

The researchers also noted that males, blacks, and Medicaid-eligible participants had higher risk estimates associated with PM2.5 exposure compared with the national average. Notably among blacks, the effect estimate for PM2.5 was three times that of the overall population.

"The message is clear," Di explained in an email: "Air pollution kills people, even below current National Ambient Air Quality Standards. Current air quality standard is not stringent enough to protect human health. We need to strengthen, not weaken, EPA air pollution standards. We need to increase, not reduce the EPA research funding."

Berger and colleagues agreed, noting in the editorial that despite compelling data such as this study, "the Trump administration is moving headlong in the opposite direction. Trump's proposed budget includes crippling cuts to the EPA, including cuts in funding for both federal and state enforcement of regulations. The increased air pollution that would result from loosening current restrictions would have devastating effects on public health."

The study was supported by grants from the Health Effects Institute, the National Institutes of Health, the National Cancer Institute, and the Environmental Protection Agency.

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